Crucible is used to melt and melt metal liquid or chemical reaction of a fire-resistant container, the general shape is like a pound garlic pot, it is big above and small below basically a cylinder, by heating the crucible can melt or extract some needed substances, is also an important tool to make some substances chemical reaction by heating.
1. Cast iron crucible
The use of iron crucible is similar to nickel crucible, it is less durable than nickel crucible, but the price is cheaper, more suitable for sodium peroxide melting, can replace nickel crucible.
Iron crucible or low silicon steel crucible should be passivation treatment before use, first soak with dilute hydrochloric acid, then wipe with fine sandpaper, rinse with hot water, then put in 5% sulfuric acid +1% nitric acid mixed solution soak for a few minutes, then wash with water, dry, at 300~400℃ burning for 10min.
2. Platinum crucible
Platinum, also known as white gold, is more expensive than gold, because it has many excellent properties, so it is often used. Platinum has a melting point as high as 1774℃, stable chemical properties, no chemical changes after burning in the air, nor does it absorb water, and most chemical reagents have no erosion effect on it.
1. Features:
The corrosion resistance of platinum to hydrofluoric acid and molten alkali metal carbonate is an important property different from that of glass and porcelain, so platinum is often used for precipitation burning weighing, hydrofluoric acid melting sample and carbonate melting treatment. Platinum is slightly volatile at high temperatures and needs to be corrected after long burning time. Platinum with an area of 100cm2 loses 1mg when burned at 1200℃ for 1h, and platinum is basically non-volatile below 900℃.
2. The use of platinum vessels shall comply with the following rules:
(1) A strict system should be formulated for the collection, use, consumption and recovery of platinum.
(2) Platinum is soft, even the alloy containing a small amount of rhodium and iridium is soft, so do not use too much force when taking platinum vessels, so as to avoid its deformation. During defrit, sharp objects such as glass rods should not be scraped from platinum vessels to avoid damage to the inner wall. Nor can you suddenly put hot platinum vessels into cold water to avoid cracks. The deformed platinum crucible or vessel can be corrected with a water mold of the same shape (but the brittle platinum carbide part should be corrected with even force).
(3) When heating platinum vessels, can not contact with any other metal, because platinum is easy to alloy with other metals at high temperature, so the platinum crucible must be placed on the platinum triangulate or ceramic, clay, quartz and other materials on the support material burning, can also be placed on the pad with asbestos plate electric heating plate or furnace heating, but can not directly contact with iron plate or electric furnace wire. Crucible pliers to be used should be covered with platinum tips, nickel or stainless steel pliers should only be used at low temperatures.
3. Cleaning method of platinum vessels:
If platinum vessels have spots, can first use hydrochloric acid or nitric acid alone treatment. If it is not effective, use potassium sulfate coke to melt in a platinum vessel at a low temperature for 5-10 min, pour the molten substance away, and then immerse the platinum vessel in hydrochloric acid solution. If still not effective, can try again sodium carbonate melting treatment, also can be used wet fine sand (through 100 mesh sieve 0.14 mm sieve) gently friction treatment.
3. Gold and silver crucible
1. Characteristics of gold crucible
The price of gold is cheaper than platinum, and is not affected by alkali metal hydroxide and hydrofluoric acid erosion, so it is often used to replace platinum vessels, but the melting point of gold is low (1063℃), so it can not withstand high temperature burning, generally must be used below 700℃. Ammonium nitrate has obvious erosive effect on gold, and aqua regia cannot contact with gold vessels. The principle of the use of gold vessels is basically the same as that of platinum vessels.
2. Characteristics of silver crucible
Silver vessels are relatively inexpensive and resistant to potassium hydroxide (sodium) erosion, which in the molten state erodes only slightly near the edge of the air. The melting point of silver is 960 ℃, the use of temperature is generally not more than 750℃ is appropriate, can not be directly heated on the fire. Heating produces a layer of silver oxide on the surface, which is unstable at high temperatures but stable below 200℃. The silver crucible, which has just been removed from the high temperature, must not be cooled with cold water immediately to prevent cracks. Silver is easy to interact with sulfur to produce silver sulfide, so it can not be decomposed and burned in the silver crucible containing sulfur, do not use alkaline sulfide reagents. Molten salts of aluminum, zinc, tin, lead, mercury, etc. can make the silver crucible brittle. Silver crucible is not used for melting borax. When using sodium peroxide flux, it should only be sintered, not fused.
Transparent quartz glass is made by high temperature melting of crystal, which is colorless and transparent. Translucent quartz is made of natural pure gangue or quartz sand and is translucent because it contains many air bubbles that have not been drained during smelting. Transparent quartz glass has better physical and chemical properties than translucent quartz, which is mainly used in the manufacture of laboratory glass instruments and optical instruments.
The thermal expansion coefficient of quartz glass is very small (5.5×10-7), only one-fifth of that of extra-hard glass. Therefore, it can be heated and cooled quickly. After burning the transparent quartz glass to red hot, it will not burst into cold water. The softening temperature of quartz glass is 1650℃, which has high temperature resistance. Quartz crucible is often used for the melting of acid flux and sodium thiosulfate, the use temperature shall not exceed 1100℃. Its acid resistance is very good, except hydrofluoric acid and phosphoric acid, any concentration of acid even at high temperature rarely interact with quartz glass. Quartz glass is not resistant to the corrosion of hydrofluoric acid, phosphoric acid can also act on it above 150℃, strong alkali solutions including alkali metal carbonate can also corrosion quartz, but the corrosion is slower at room temperature, and the corrosion is accelerated when the temperature rises. The appearance of quartz glass instrument is similar to that of glass instrument, colorless and transparent, but it is more expensive than glass instrument, more brittle and easy to break. Special care should be taken when using it. It is usually stored separately with glass instrument and properly kept.
4. Corundum crucible
Natural corundum is almost pure alumina. Artificial corundum is made of pure alumina by high temperature sintering, it is high temperature resistant, melting point is 2045℃, high hardness, acid and alkali have considerable corrosion resistance.
5. Ceramic crucible
The porcelain dish used in the laboratory is actually glazed pottery. It has a high melting point (1410℃) and can withstand high temperature burning. For example, the porcelain crucible can be heated to 1200℃, and its mass changes little after burning, so it is often used for burning and weighing precipitation. High - shape porcelain crucible can handle samples in air - free condition.
The thermal expansion coefficient of the porcelain dishes used in the laboratory is (3~4) ×10-6. In the process of evaporation and burning at high temperature, the sudden change of temperature and uneven heating should be avoided to prevent the cracking of thick walled porcelain dishes. Porcelain dishes are more stable to chemical agents such as acid and alkali than glassware, but they also cannot contact hydrofluoric acid. Porcelain crucible is not resistant to caustic soda and sodium carbonate corrosion, especially can not carry out their melting operation. Some non-porcelain materials such as MgO and C powder are used as fillers to melt silicate samples in a porcelain crucible with quantitative filter paper wrapped in alkaline flux, which can partially replace platinum products. Porcelain dishes have strong mechanical properties and are cheap, so they are widely used.
6. Graphite crucible
Graphite crucible is based on natural graphite as the main material, with high-temperature clay as the binder, but also add asphalt and tar and other raw materials, mainly used for smelting special alloys and melting non-ferrous metals. The main material of graphite crucible is graphite, so it has many advantages and characteristics of graphite, such as can withstand the high temperature of 1200-1600 degrees Celsius, heat transfer performance is very strong, which can effectively reduce the melting time, reduce energy consumption and improve efficiency.
Graphite crucible has relatively stable chemical properties, which makes it not react with metal solution in the smelting process. The inner and outer walls of the graphite crucible are very smooth, which leaves no metal solution and residue on the surface, so that if you pour water on the graphite crucible, not a drop of water will get on the surface. At present, our country's production technology level of graphite crucible already exceeds even the level of imported crucible.
